NEW YORK -- An hour into a performance defined by its adulation, Troye Sivan welcomed his opener Dua Lipa back to the stage. The two electro-pop newcomers, neither of whom had cracked the U.S. consciousness this time last year, sang together before 3,000 shrieking teen fans Tuesday night, unveiling a magnetic duet of Mark Ronson's bump-and-goâ€¦